"It was a really tough battle today," said Wawrinka, who also survived a tumble into the courtside seats in Armstrong Stadium when he hit the knee-high board surrounding the court at full stretch for a shot.

"I had some ups and downs physically," he said. "From that I'm OK, I'm more tired from the running, from the match.

"When you win, it's always more easy to feel good after," added Wawrinka, who had won only two of his prior eight matches against Robredo.

One of those victories was in the fourth round of the Australian Open en route to his first Grand Slam title.

He will face either fifth-seeded Canadian Milos Raonic or 10th seeded Kei Nishikori of Japan for a place in the semi-finals.